378 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol.XVIU. 



carapace you have cut. Separate the mandibles from one another; 

 then with forceps pull the cut end of the muscle and observe how 

 the mandibles are adducted or brought together in the middle 



line. ; 



The Alimentary System. 



The jaw apparatus has already been described and the student will 

 have noted that the jaws are all external to the oral opening and riot, 

 as in the Vertebrates, inside it. 



The mandibles have been noted as enormous in size even for a 

 Crustacean. Lobsters, Prawns and Crabs have large forceps, by means 

 of which they can crush and tear their food before swallowing it. 

 Palinurus, not being thus gifted, has some compensation in the size 

 and power of the mandibles. 



The mouth, we saw, was flanked by the mandibles and bounded 

 in front by the fleshy labrum, behind by the metastoma. It opens 

 into a short wide gullet or oesophagus, running nearly vertically 

 upwards to enter the large gizzard or stomach which occupies the 

 greater part of the head cavity. 



We may here remind you that in the embryo of the higher 

 animals the primitive gut or alimentary canal is lined by the inner of 

 the three embryonic layers, the hypoblast or endoderm. The original 

 opening into this gut becomes completely closed and it is only at a 

 later stage that the mouth and anal openings are formed. They are 

 formed by a dimpling or tucking in of the outer layer, the epiblast or 

 epiderm, till it meets the hypoblast, and an opening is made where they 

 touch. This tucking in or invagination is very shallow in most animals, 

 much the greater portion of the digestive canal being lined by the 

 hypoblast, only a very short distance inside the lips and anus by 

 epiderm. In other words, the adult canal is mainly a development of 

 the primitive gut. 



In the Arthropods we have a striking contrast. The dimples iri 

 front and behind, known respectively as the st&modceum (Gr. stoma, 

 mouth, odaion, a passage) and proctodeum (Gr. proktos vent, anus) 

 become deeper and deeper till instead of being mere pits they form 

 long tubes that eventually meet the very short primitive gut or 

 mesenteron (Gr. mesos, middle; enteron, gut). 



In Palinurus this mesenteron is less than a twentieth I of the whole 

 length of the alimentary canal. The oesophagus and stomach are 



